1. Field of the Invention
This invention is related in general to the field of optical detectors. In particular, it relates to a variable-focus infrared optical detector system used for temperature measurements.
2. Description of the Related Art
Infrared (IR) detectors are utilized in various applications such as imaging, detection of spectral signatures of chemical species, and remote sensing. In addition, IR detectors are known to be used as IR thermometers (IRTs). The active surface of a typical IR-detector working as a thermometer consists of a series of thermocouple junctions which produce an output voltage in response to IR energy collected and delivered to the detector by the IRT's optical system. Since any target emits IR energy in proportion to its temperature, the output reading of IRTs is directly proportional to the target's temperature as well.
IRTs are known to be extremely susceptible to reading errors resulting from improper alignment with the target. An accurate temperature reading of the target can be made only if the area imaged upon the surface of the IR detector completely resides within the target boundaries. Since the background to any target under test emits a certain amount of IR radiation, when the IRT's optical system is not pointing directly at the target, the detector collects energy coming from not only the target but also from background, thereby providing an erroneous output reading. This shortcoming of conventional IRTs is augmented by the fact that IRTs operate in the portion of light spectrum not visible to the human eye (typically at wavelengths longer than 2 μm) and, therefore, the alignment of the IRT with the target cannot be visually controlled.
FIG. 1 illustrates schematically such a conventional condition. An optical system L collects IR energy emitted by a measured area MA that depends on the geometry and alignment of the system with a target T. The boundaries of MA are not visible to the operator and, therefore, are not easily identified. As a result, when the optical system L is misaligned with respect to target T, the IR energy incident on the detector D from within the area MA and giving rise to the output voltage V may be received not only from an area A on the target but also from a background area B outside the target (the areas A and B together comprising the measured area MA). Therefore, the output reading of the IR detector that is specifically calibrated to measure the temperature of the surface of the target Twill contain an error. This problem persists regardless of the imaging conditions provided by the optical system L of the IRT. For example, if a zoom lens is used, a reading error will be present for any focal setting of the lens as long as the area MA imaged onto the detector D does not completely fall within the perimeter of the target T. As is well understood in the art, additional reading errors may also be introduced when the target area under test is tilted with respect to the optical axis of the IRT system.
An attempt has been made to solve the problem of determining the exact position of the target area under test by incorporating a source of visible laser radiation (such as helium-neon laser) into the IRT system, as shown in FIG. 2. In such configuration, the laser beam LB is typically aligned with the optical axis of the IRT system and delivered to the target through a lens L, thus visibly identifying the center C of the measured area MA. This solution does not solve the major problem, however. While spotting the position of the center of the measured area inside the perimeter of the target T, it does not show the boundaries of the measured area. Consequently, the IR detector will still collect background radiation when the measured area MA falls partly outside the perimeter of the target T due to misalignment of the optical system L with respect to the target.
Therefore, any IRT system that provides for unambiguous alignment of the IRT with respect to the measured target would represent a very desirable advance in the art, especially under variable imaging conditions. This invention achieves this goal by using a system of target sighting in the visible spectral region that is adapted to unambiguously identify not only the position but also the size of the target area imaged on the IR detector under variable focus conditions.